Method for communicating in a mimo context

ABSTRACT

A primary station includes an array of transmit antennas to communicate with at least one secondary station on a downlink channel; at least one processor coupled to a memory, the at least one processor being configured to: generate a pre-coding to be applied during a corresponding transmission to the at least one secondary station for each transmit antenna in the array; apply a reversible transform to the pre-coding, the pre-coding being independent of data to be transmitted to the secondary station during said corresponding transmission to the secondary station so as to generate a set of pre-coding coefficients representative of the pre-coding in a transform domain; generate a set of parameters comprising at least one parameter indicative of the set of pre-coding coefficients; signal the set of parameters to the secondary station; and transmit data to the secondary station in accordance with the pre-coding.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. Ser. No. 12/597,013 filed on Oct. 22,2009, pending, which was the national stage of international patentapplication no. PCT/IB2008/051630 filed on Apr. 28, 2008, which claimspriority to European patent application no. EP 07301010.0 filed Apr. 30,2007. The entire disclosures of the foregoing applications areincorporated herein by reference. The international patent applicationpublished in English on Nov. 6, 2008 as WO/2008/132688.

TECHNICAL FIELD

The present disclosure relates to a method for communicating within anetwork.

BACKGROUND

Multiple-input multiple-output (MIMO) is a technology for nextgeneration wireless systems to enhance the capacity and robustness ofthe communication link. MIMO technology is based on the presence ofmultiple transmit antennas and multiple receive antennas in thecommunication link. Application of MIMO technology is envisioned forcellular communication, broadband wireless access, as well as forwireless local area networks (WLANs). A plurality of two or moretransmit antennas is also referred to as an array of transmit antennasherein.

The benefits of MIMO communication are obtained through a combination ofantenna arrays that provide spatial diversity from the propagationchannel and algorithms that can adapt to the changing multivariatechannel.

In future mobile systems and in the long-term evolution of the UniversalMobile Telecommunication System (UMTS LTE) the use of multiple-antennatechniques will become increasingly important to meet spectralefficiency requirements. A significant gain in spectral efficiency canbe achieved in a downlink transmission by multiplexing multiplecodewords in the spatial domain to either a single user or multipleusers sharing the same time-frequency resource block. These single-useror multi-user MIMO schemes exploiting the multiplexing gain ofmulti-antenna transmission are sometimes referred to as spatial divisionmultiplexing (SDM) and spatial division multiple access (SDMA)techniques. An SDMA scheme enables multiple users within the same radiocell to be accommodated on the same frequency or time slot. Therealization of this technique can be accomplished by using an antennaarray, which is capable of modifying its time, frequency, and spatialresponse by means of the amplitude and phase weighting and an internalfeedback control.

Beamforming is a method used to create a radiation pattern of theantenna array by constructively adding the phases of the signals in thedirection of the communication targets (terminal devices) desired, andnulling the pattern of the communication targets that are undesired orinterfering.

In this context, the beamforming vector plays an important role. Forpurposes of illustration of the meaning of the beamforming vector, in anexemplary single-user communication system employing transmitbeamforming and receive combining, assuming that signaling is done usingM transmit and N receive antennas, the input-output relationship of thiscommunication system is given by

y=z ^(H) Hwx+z ^(H) n

where H is a N×M channel matrix connecting the transmitter and thereceiver, z is the receive combining vector, z^(H) is its Hermitiantranspose, w is the transmit beamforming vector, x is the transmittedsymbol from a chosen constellation, and n is independent noise added atthe receiver.

The aim is to design the signal x such that it can effectively conveyinformation to the users.

One of the challenges in the design of the beamforming vectors for SDMand SDMA techniques is the need for the base station to know thechannels for all the users and receiving antennas of each user. Thiswould require a large amount of feedback to be signaled from the usersto the base station.

Solutions have been proposed to reduce this signaling information byintroducing a codebook of few possible beamforming matrices. Each userthen applies a greedy procedure to select one or more preferredbeamforming vectors out of the codebook, by evaluating theSignal-to-Noise-Ratios (SINRs) of different beamforming combinations.Thus, each user has to signal one or several indexes of the preferredvector or vectors, respectively, plus one or moreChannel-Quality-Indicator (CQI) values, indicating the correspondingSINRs.

An issue with codebook-based solutions is that the beamforming vectorsare not jointly optimized according to the channel conditions. The basestation uses the feedback information from the users only to scheduletransmission to the set of users reporting the best CQI values.

Alternatively, significant gain in the cell throughput can be achievedif the base station could implement an ad-hoc design of the beamformer.This is possible, for example, if the users report all the channelcoefficients, after some quantization operation. However, this requiressignaling as many complex values as the product, MN, between the numberM of transmit antennas and the number N of receive antennas per user.

Similarly, a solution to this problem based on channel quantization andzero forcing beamforming is described in “Transform-Domain FeedbackSignalling for MIMO Communication” in the applicant patent applicationhaving reference PH006732EP4.

Another approach is PU2RC (per-user unitary rate control)

In FIG. 1 a block diagram is drawn of the fundamental operations carriedout at the transmitter (Node B) and receivers (UE's) in a typicalmulti-user downlink MIMO scheme. The method described here is used toreduce the number of bits required to signal the precoding matrix U fromthe Node B to each UE selected for transmission.

FIG. 1 is a block diagram of the downlink multi-user MIMO operationscarried out at the transmitter and receivers. The AMC block performsadaptation of modulation and coding for each spatial stream to betransmitted. The feedback from each UE consists of a PMI (precodingmatrix indicator) index selected from a codebook of vectors and areal-valued CQI (channel quality indicator), which is an estimate of theSINR for the relevant received spatial stream.

The feedback from the Node B conveys quantised information on thevectors forming the precoding matrix U. This feedback from the Node B isthe subject of the present disclosure.

Many solutions are known for pre-coding (also designated as beamforming) for Multi-user MIMO systems where pre-coding vectors arecomputed for streams transmitted to each of several users. However, itis desirable to be able to indicate to the receiver the beamformingcoefficients which are used at the transmitter. In principle, for bestperformance, each user should know both its own coefficients as well asthose for users.

There is a need to efficiently signal the precoding coefficients to eachof the users scheduled for transmission so that they can derive asuitable phase reference for their wanted signal (e.g. from commonreference signals), and also preferably to be able to optimise receivercoefficients possibly taking into account interference from signalsintended for other users.

SUMMARY

Proposed herein is an efficient method for signaling the vectors ofprecoding without creating too much overhead on the signaling channel.To this end, proposed herein is a method for communicating from aprimary station with an array of transmit antennas to a secondarystation on a downlink channel, said method comprising steps of, at theprimary station,

(a) configuring the downlink channel,

-   -   step (a) being subdivided into steps of:    -   (a2) computing a precoding to be applied during a corresponding        transmission from the primary station to the secondary station        for each of an array of transmit antennas;    -   (a3) applying a reversible transform to the precoding, thus        ascertaining a set of precoding coefficients indicative of the        precoding in a transform domain;    -   (a4) computing a set of parameters comprising at least one        parameter, said parameter being substantially representative of        the coefficients obtained at step (a3);    -   (a5) signaling the set of parameters to the secondary station;

(b) transmitting data to the secondary station substantially accordingto the precoding computed at step (a2).

The present disclosure is based on the realisation that any welldesigned set of beamformers is likely to have low cross correlation (oreven approach orthogonality). This means that the primary requirement isto be able to signal the coefficients for the beamformer used for thewanted signal.

As a consequence, the method of the present disclosure allows anefficient signaling of the beamforming process on the transmitter side,based on communicating a set of precoding coefficients indicative ofprecoding in a transform domain from the transmitter to the receiver.

Moreover, as another aspect of the disclosure that can be combined withthe first aspect, one can consider the downlink signaling itself.

In order to build the optimum receiver, it is necessary to know theprecoding vectors used by the base station for allsimultaneously-scheduled users. This enables a receiver with multiplereceive antennas to derive the phase reference for its own data as wellas to compute combining coefficients for rejection of interference fromthe signals for other users.

However, this results in a very large downlink signaling overhead.

According to this aspect, it is recognized that the most importantpurpose of the above downlink signaling (e.g. in terms of improvingperformance or total throughput) is providing sufficient information forthe receiver to derive the precoding vector used for its own data.Further improved performance may be gained by interference rejectionusing some knowledge of the precoding vectors for other users, but thisis not the primary goal that the said downlink signaling mustaccomplish.

Therefore according to the signaling aspect, the amount of informationprovided pertaining to the precoding vector of the user's own data isgreater than the amount of information provided pertaining to othersimultaneously-used precoding vectors.

The former said amount of information may for example be greater interms of having a finer resolution, more frequent update rate, finerfrequency-domain granularity, etc.

Also disclosed herein is a base station adapted to implement the methodof the first aspect of the disclosure.

These and other aspects will be apparent from and will be elucidatedwith reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure will now be described inmore detail, by way of example, with reference to the accompanyingdrawings, wherein:

FIG. 1 is a block diagram representing the fundamental operationscarried out at the transmitter (Node B) and receivers (UE's) in atypical multi-user downlink MIMO scheme.

DETAILED DESCRIPTION

The present disclosure relates to a method of communicating from aprimary station to a secondary station. Such a primary station can be abase station or a NodeB.

Such a secondary station can be a mobile station or a user equipment.

1) In one embodiment, the cellular system uses TDMA. The radio channelis assumed to be non-dispersive (i.e. subject to flat fading). Eachmobile terminal has a single receive antenna. Each base station has aplurality of transmit antennas, (as an example let assume 4 transmitantennas). Orthogonal pilot signals are transmitted from each antenna.Frequency re-use is applied so that each cell uses a different frequencyfrom its neighbours. Each mobile terminal (or user) is allocated to acell served by a particular base station. The following procedure isapplied:

The network receives measurements from the mobile terminals on thetransfer function of the downlink channel from each of the transmitantennas of the base station serving its cell. Indeed, the base stationreceives from each secondary station channel state information, that canbe signaled on a signaling channel. This information can be an estimateof a preferred direction for transmission, or as processed in the patentapplication with the applicant's reference PH006732EP4.

The network can make a selection of the mobile terminals served by eachcell, based for example on transfer functions, and availability andpriority of data to be transmitted. Indeed, the information included inthe channel state information obtained in the previous step can be used.This selection can depend on several criterion. For instance, it ispossible to make this selection based on the channel state informationreceived from the secondary stations. Moreover, alone or in combinationwith the preceding criterion, the selection can be made depending on thepriority of the data to be transmitted. Another example is to select afirst station based on a criterion like discussed above, and thenselecting secondary station whose channel properties are not highlycorrelated with the firstly selected secondary station. Still anotherpossibility is to select the secondary stations to obtain the maximalthroughput.

Pre-coding coefficients are then computed for each scheduled user, forexample using zero forcing beamforming The coefficients are notmandatorily corresponding to the preferred directions and otherinformation signaled by the secondary station previously. In fact, thesecoefficients can be different from the signaled coefficients, and arebased on the computation of the primary station, but this computationcan be carried out at least partly with help of the respective channelstate information received previously at the base station.

A reversible transform, for instance linear and orthogonal, like an IDFT(Inverse Discrete Fourier Transform), is carried out for translating theprecoding into coefficients in a transform domain, like the angulardomain for the example of IDFT. Thus, a set of precoding coefficientsindicative of the precoding in the transform domain is obtained. Thesecoefficients can be quantized for problems of reducing data to betransmitted.

An index can be deduced for instance by comparing the values of thecoefficients with look up tables. It is for instance possible to comparethe maximum value of the set of coefficients to select a table, whoseindex would then be transmitted to the selected secondary stations inthe following step.

Information identifying the users expected to receive data, thetransmission format and any other information needed to receive the datais sent. This would include an index representing or indicating thepre-coding coefficients applied for each user, as seen above.

Data is then transmitted to each of the selected users using thepre-coding coefficients and some of the available transmit antennas.Data for different users may be sent at the same time.

With 4 transmit antennas, simultaneous transmissions to up to 4 users ispossible.

The step of computing the precoding may be carried out with target toobtain the highest possible data rate.

2) In another embodiment otherwise like (1), the cellular system usesOFDM, and the radio channel is assumed to be dispersive, but can beconsidered as flat within each sub-carrier. Let us assume 32sub-carriers. Other differences with the first embodiment are as follows

The mobile terminal makes measurements of the downlink transfer functionfor each sub-carrier and each transmit antenna.

The final user selection includes an allocation of sub-carriers for eachuser.

The allocation of sub-carriers is preferably signaled to the mobileterminals.

In principle, with 4 transmit antennas and 32 sub-carriers transmissionscould be made to 128 users simultaneously. However, in a practicalsystem this number would probably be lower.

3) In another embodiment otherwise like (1) a cellular system uses CDMA(such as UMTS). The radio channel is assumed to be non-dispersive. Thebase stations are distinguished by different scrambling codes.Orthogonal pilot sequences are transmitted from each antenna. Otherdifferences with the first embodiment are as follows:

Different channelization codes may be used to provide orthogonalcommunication channels, but the channel transfer function does notdepend on the channelization code.

The final selection user selection may include allocation ofchannelization codes to users. However, this would need to be done onthe basis of scheduling principles rather than be determined by theradio channel transfer function

Data for different users may use the same channelization code.

With 4 transmit antennas, simultaneous transmissions to up to4×number_of_channelization_codes is possible.

4) In another embodiment otherwise like (1), each mobile terminal hastwo antennas. In this case up to two data streams may be transmitted toa user. In this case the transmitter may signal two pre-coding indices,to the user, representing the each of the transmitted beams.

5) In another embodiment, otherwise like (1) the system uses TDD. Underthe assumption of channel reciprocity, the channel transfer functions inone direction can be determined by observing signals transmitted in theother direction.

In general, for dispersive channels, the pre-coding used at thetransmitter may vary as a function of frequency (e.g. with differentvectors applied to different parts of the spectrum). In such a case, itmay be desirable for the transmitter to be able to signal more than onedifferent pre-coding vectors.

A different vector/scalar quantization technique, other than theIDFT-based quantization could be used.

In one embodiment, transmissions take place to multiple userssimultaneously using different precodings, and the time-frequencyresource blocks assigned for transmissions to the different users may begrouped differently. In such a case, the provision of complete precodinginformation to each user in respect of the precoding used for everyother user whose time-frequency resources wholly or partially overlapwith the user's own resources would result in a very high signalingoverhead. By transmitting a reduced amount of information about theother users' precoding, it can be designed to be valid for more than oneother user.

FIELDS OF APPLICATION

Radio communication systems, especially mobile and WLAN systems. Inparticular cellular systems such as UMTS and UMTS LTE.

1. A primary station comprising: an array of transmit antennas tocommunicate with at least one secondary station on a downlink channel;at least one processor coupled to a memory, the at least one processorbeing configured to: generate a pre-coding to be applied during acorresponding transmission to the at least one secondary station foreach transmit antenna in the array; apply a reversible transform to thepre-coding, the pre-coding being independent of data to be transmittedto the secondary station during said corresponding transmission to thesecondary station so as to generate a set of pre-coding coefficientsrepresentative of the pre-coding in a transform domain; generate a setof parameters comprising at least one parameter indicative of the set ofpre-coding coefficients; signal the set of parameters to the secondarystation; and transmit data to the secondary station in accordance withthe pre-coding.
 2. The primary station of claim 1, wherein the at leastone processor is further configured to receive channel state informationfrom a secondary station.
 3. The primary station of claim 2, wherein theat least one processor is configured to generate the pre-coding based atleast partially on the channel state information.
 4. The primary stationof claim 1, wherein the at least one processor is configured to generatethe pre-coding so as to maximize a data rate.
 5. The primary station ofclaim 1, wherein the at least one processor is further configured toselect the secondary station from a plurality of secondary stations. 6.The primary station of claim 5, wherein the at least one processor isconfigured to select the secondary station from the plurality ofsecondary stations based at least partially on an indication of achannel quality of each secondary station.
 7. The primary station ofclaim 5, wherein the at least one processor is configured to select thesecondary station from the plurality of secondary stations based atleast partially on an indication of a priority of data to be transmittedto each secondary station.
 8. The primary station of claim 5, wherein toselect the secondary station the at least one processor is configuredto: select a first secondary station; and select further secondarystations having channel properties that are not highly correlated withthose of the first secondary station.
 9. The primary station of claim 1,wherein the at least one processor is configurede to select thesecondary station such that a total data throughput is maximized. 10.The primary station of claim 1, wherein the reversible transform islinear and orthogonal.
 11. The primary station of claim 10, wherein toapply the reversible transform to the pre-coding the at least oneprocessor is configured to apply an Inverse Discrete Fourier transformto the set of precoding coefficients.
 12. The primary station of claim10, wherein the set of parameters is based on a co-efficient from theset of precoding coefficients in an angular domain having a maximummagnitude.
 13. A secondary station comprising: an array of receptionantennas for communicating with a base station on a downlink channel; atleast one processor coupled to a memory, the at least one processorbeing configured to: determine channel state information; transmit thechannel state information to the base station; receive a set ofparameters from the base station comprising at least one parameter, saidat least one parameter being representative of a set of pre-codingcoefficients in an angular domain obtained by applying a reversibletransform to a pre-coding, wherein said pre-coding is independent ofdata to be received by the secondary station, the pre-codingcoefficients being based at least partially on the channel stateinformation; derive a phase reference depending on said set ofparameters; and receive data from the base station according to thephase reference.
 14. The secondary station of claim 13, wherein thechannel state information comprises an estimate of a preferred directionof a transmission beam from the base station.
 15. The secondary stationof claim 13, wherein the at least one processor is further configured tomeasure a downlink transfer function for each sub-carrier and eachtransmit antenna of the base station.
 16. A base station comprising: amemory; an array of transmit antennas to communicate with user equipmentover a downlink channel; at least one processor configured to: generatepre-coding coefficients for each transmit antenna in the array oftransmit antennas, the pre-coding coefficients being based at leastpartially on channel state information associated with the userequipment; translate the pre-coding coefficients into a set ofpre-coding coefficients indicative of pre-coding in an angular domain;generate indices associated with the set of pre-coding coefficientsindicative of pre-coding in the angular domain; transmit at least theindices to the user equipment; and transmit data to the user equipmentin accordance with the set of pre-coding coefficients associated withthe indices.
 17. The base station of claim 16, wherein the channel stateinformation comprises measurements associated with a transfer functionof a downlink channel from each transmit antenna in the array oftransmit antennas.
 18. The base station of claim 16, wherein the atleast one processor is further configured to select at least one userequipment for communication over the downlink channel.
 19. The basestation of claim 18, wherein the at least one processor is furtherconfigured to select the at least one user equipment based at leastpartially on channel quality.
 20. The base station of claim 19, whereinthe at least one processor is further configured to select the at leastone user equipment based at least partially on a priority of data to betransmitted to the at least one user equipment.
 21. The base station ofclaim 16, wherein the at least one processor is configured to employzero forcing beam forming to generate the pre-coding coefficients foreach transmit antenna.
 22. The base station of claim 16, wherein the atleast one processor is further configured to utilize a reversibletransform to translate the pre-coding coefficients into coefficients inthe angular domain.
 23. The base station of claim 22, wherein thereversible transform is liner and orthogonal.
 24. The base station ofclaim 16, wherein the at least one processor is further configured toutilize Inverse Discrete Fourier Transform to translate the pre-codingcoefficients into coefficients in the angular domain.
 25. A mobilestation comprising: a memory; a plurality of antennas to communicatewith a base station over a downlink channel; at least one processorconfigured to: transmit channel state information to the base station;receive indices from the base station, the indices being associated witha set of pre-coding coefficients in an angular domain obtained byapplying a reversible transform to initial pre-coding coefficients, theinitial pre-coding coefficients being independent of the set ofpre-coding coefficients; derive a phase reference based at leastpartially on the indices; and receive data from the base station inaccordance with the phase reference.
 26. The mobile station of claim 25,wherein the channel state information comprises an estimate of apreferred direction of a transmission beam from the base station. 27.The mobile station of claim 25, wherein the at least one processor isfurther configured to measure a downlink transfer function for eachsub-carrier and each transmit antenna of the base station.